>
>
>Dear Martin,
>
>We need an opinion from you.
>
>In the production, we have problems with the SMD tantulum capacitors from
>the aspects of cost and the lead-time.
>
>Our Taiwan people have suggested a new family of electrolytic caps named
as
>"PETIT" which is also SMD type but costs very less. It has less lead time.
>But the only obvious problem is it has slightly bigger foot-print and
>higher. Please refer to the attached data sheet for PETIT.
>
>Although the PETIT family is cheap and available in short time, the PCB
pad
>size seems to be bigger, and we can not use these caps in certain areas
>where we have height restrictions.
>
>Then we found another alternate part from AVX. It is of Ceramic type with
>Y5V dielectric. In which we have caps from 2.2 to 22uF. The pages 16 to 18
>of the attached PDF document shows the Y5V data. The attached GIF file
>shows the parts of our interest. Most of our appications may exploit these
>caps. The size of these caps is 1206 which is quite smaller than the
>tantulum and the PETIT caps.
>
>While we are going to analyze the cost of these AVX ceramic, could you
>please let us know what will be the implications of Ceramic caps in places
>of Tanatulum and PETIT-Electrolytic? The question is, with reference to
the
>attached data sheets, would it be good to use the Ceramic parts in places
>of the tantulums on DC Power planes?
>
>We have done some studies and found the following differences:
>
>(1) The Tantulum seems to have low impedences at high frequencies; then
>comes the PETIT (electrolytic) which have slightly higher impedences and
>then the Ceramic that has further high impedences at high frequencies.
>
>(2) The Ceramic caps are not polarized; whereas the tantulum and
>electrolytic are polarized.

February 17, 2000

Tantalum has always been expensive and rare. In addition, such capacitors
need to be rated at twice the supply voltage for best reliability (don't
ask me why - it's just one of those things that was drummed into me at an
early age!)

So any alternative to tantalum has to be better, from the point of view
of cost and availability, although tantalum is my favourite for
performance and reliability.

I am not happy with wet electrolyte aluminium foil capacitors. I have no
experience of the "Petit" type but I know that surface mount
electrolytics in camcorders are proving to be very unreliable after 2 - 3
years of use. The rating of only 2000 hours at 85 or 105'C is not very
impressive. I would not expect a camcorder to become very warm inside. I
wonder what temperatures your equipment sees?

I am not happy with multilayer ceramic capacitors. The values of 47nF and
higher seem especially prone to flame problems when connected to a 5 volt
supply. However, there is no electrolyte to evaporate and the life will
be infinite, provided that the capacitor is not cracked. I am very
surprised that you find higher impedances with the ceramic than with the
aluminium electrolytic "petit" type. I think you will find very much the
reverse is true after a few months of use.

However, in my experience many designers use a "belt and braces"
technique by asking for as many DC power plane decoupling capacitors as
space will allow! They do not seem to acknowledge that the ICs which take
the most power and have the fastest risetime pulses need better
decoupling than ICs which are low power CMOS, for example. Sometimes no
decoupling is required at all. It is always best to measure a production
board in order to optimise the decoupling. Of course there is never time
to do the design correctly. As the American saying goes: "There's never
time to do it right, but there's always time to do it over." The design
*always* gets completed by Production engineers, in my experience.

Whatever you decide to use, you will have to carry out tests for
performance and reliability. Putting the equipment inside a chamber with
fairly high humidity and temperature cycling up and down will help to
find most problems in just 2 or 3 days. The equipment should be *running*
during the test and, of course, the temperature and humidity must be kept
within reasonable limits. The idea is to make it "breathe" - not to
stress it beyond reasonable limits. You don't test a camel to see if it
will survive in the desert by sticking a red hot poker up its backside!

Performance tests should be easy to carry out. It needs only a high
frequency oscilloscope to view the spurious pulses on the DC power planes
and to decide whether they are below the limits which will cause
problems. Of course, the equipment also needs to meet emission standards
as well, so this should be a consideration. I would firstly compare a
"standard" board assembly with one that has the new capacitors fitted.

If you decide to use ceramic capacitors, do bear in mind the ways to
minimise stress, as discussed in my web site pages. If you decide on wet
electrolytics then remember that they degrade fairly quickly at
temperatures above 20'C.

OK, I am sorry that I can not give you a simple "yes" or "no" answer but,
as you know, there are several factors to be considered.

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